What is the initial diagnostic study for urinary tract evaluation in a patient with blood at the external urethral meatus?

The initial diagnostic study for urinary tract evaluation in a patient with blood at the external urethral meatus is retrograde urethrography. This imaging technique is particularly important because the presence of blood in the urine can suggest a potential injury to the urethra, especially in trauma cases.

Retrograde urethrography allows for visualization of the urethra and can help detect any strictures, lacerations, or other injuries. It is performed by injecting contrast material into the urethra and taking X-rays to evaluate the urethral anatomy. This approach helps to ascertain the extent of any injury before proceeding with further interventions or invasive procedures such as cystoscopy, which could exacerbate an existing injury.

Choosing retrograde urethrography as the initial diagnostic tool is crucial because it aids in identifying potential urethral injuries that could be life-threatening if not addressed promptly, allowing for appropriate management of the patient based on the findings. Other imaging studies like cystoscopy, cystography, and IV pyelography are more appropriate for other forms of urinary tract evaluation but are not the best initial choices in the context of suspected urethral trauma due to the risk they pose.